1. Technical Field of the Invention
The present invention relates to a composite rare-earth anisotropic bonded magnet having both excellent magnetic properties and extremely low aging loss, a compound employed in that magnet, and methods for their production.
2. Background Art
In recent years, with the increasing need for various types of motors and magnetic actuators with higher performance/smaller size, an improvement in the magnetic properties used in these motors and magnetic actuators has been sought. Above all, there is a strong need for higher-specification rare-earth magnets with outstanding magnetic properties. In particular, performance improvements in rare-earth anisotropic bonded magnets, which possess the traits of high size-accuracy and integral molding, have been strongly sought.
The magnetic properties and heat resistance of rare-earth anisotropic bonded magnets (hereafter, “bonded magnets”) will be explained below.
At present, RFeB rare-earth magnets comprised of rare-earth elements (R), boron (B), and iron (Fe) are being actively developed in the search for better magnetic properties. For example, RFeB magnetic alloys (composition) having magnetic isotropy were made public in patent document 1 (U.S. Pat. No. 4,851,058) and patent document 2 (U.S. Pat. No. 5,411,608), applications dated about twenty years ago.
However, conventional rare-earth magnets easily deteriorate, due to the oxidation of R and Fe which are their main ingredients, and their initial magnetic properties are not stable over time. In particular, when using rare-earth magnets above room temperature, magnetic properties decline. Ordinarily, aging loss is quantitatively indicated by the irreversible loss rate (%). The irreversible loss rate is the loss of magnetic flux which can not be recovered even after remagnetizing, following the passage of a long period of time (more than 1000 hours) at high temperature (100° C. or 120° C.). The irreversible loss rate of most conventional rare-earth anisotropic magnets is more than −10 percent.
Also, when producing rare-earth anisotropic bonded magnets from the magnet alloys made public in patent documents 1 or 2, it is necessary to confer anisotropy by crushing a magnet alloy made via melt spinning method, and then hot-pressing the crushed material. However, the magnetic properties of that magnet powder are low, and therefore the magnetic properties of bonded magnets obtained from that powder are naturally inadequate.
Aiming for further improvement in the magnetic properties of bonded magnets, the below-mentioned patent documents 3-11 propose a molded bonded magnet made by mixing magnet powder which has a plurality of different grain diameters with a binding resin. In this bonded magnet, because magnet powder with a small grain diameter enters into the empty gaps of a magnet powder with large grain diameter, the filling factor (relative density) for the whole is high, and magnetic properties are excellent. In particular, the composite rare-earth anisotropic bonded magnet, in which anisotropic magnet powder is molded within a magnetic field, manifests outstanding magnetic qualities. Below, the bonded magnet made public in each patent document will be individually explained.
In patent document 3 (Japanese patent application Laid-Open (Kokai) No. 5-152116), a bonded magnet is made public in which an epoxy binder resin is added to a mixture of magnet powder combining, in a wide variety of ratios, magnet powder made from an Nd2Fe14B alloy and having a grain diameter of 500 μm or less (hereafter, “NdFeB magnet powder”), and magnet powder made from an Sm2Fe17N alloy and having a grain diameter of 5 μm or less (hereafter, “SmFeN magnet powder”). The mixture is molded in a magnetic field, and the resin is then heat-hardened. This composite rare-earth anisotropic bonded magnet, by improving the filling factor of the whole, has a maximum energy product (BH)max of 128 kJ/m3, improving magnetic properties over bonded magnets made from simple NdFeB magnet powder whose maximum energy product (BH)max is 111 kj/m3. The grain diameter of NdFeB magnet powder was decided after carefully considering that magnetic properties deteriorate when the Nd2Fe14B alloy is simply fine ground, and the grain diameter of SmFeN magnet powder was decided after carefully considering the single domain particle coercive force structure of SmFeN magnet powder.
In patent document 4 (Japanese patent application Laid-Open (Kokai) No. 6-61023), a composite rare-earth anisotropic bonded magnet is made public in which a mixture of SmFeN magnet powder, SmCo magnet powder, and/or NdFeB magnet powder, and a lubricant or coupling agent and epoxy resin is press molded within a magnetic field. The contents of this disclosure, except for the point of using a coupling agent, do not differ greatly from the above-mentioned patent document 3. Specifically, the maximum energy product (BH) of this bonded magnet is not more than about 110 kJ/m3. In addition, in patent document 3 and patent document 4, only the magnetic properties are disclosed; nothing is recited with respect to those magnets' heat resistance or irreversible loss rate.
In patent document 5 (Japanese patent application Laid-Open (Kokai) No. 6-132107) as well, just as in above-mentioned patent document 3, a bonded magnet is disclosed which molds a mixture of NdFeB magnet powder, SmFeN magnet powder, and binder resin within a magnetic field. However, in this patent document, nothing is concretely disclosed concerning the magnetic properties or production process of the magnet powder, which exert a large influence on the magnetic properties of the bonded magnet. The maximum energy product (BH)max of the bonded magnet mentioned in the example embodiment is as much as 239 (30.3 MGOe) kJ/m3, but considering the level of technology at the time of the application, that manner of unusually high magnetic properties is not possible. Accordingly, the credibility of the data disclosed in patent document 5 as a whole is very low. For example, in chart 1 of patent document 5, looking at the value of Br for each sample, a (BH)max value equivalent to the theoretical value has been cited.
Additionally, the (BH)max value of sample no. 22 exceeds the theoretical value by 0.5 MGOe. Making an actual calculation, the value of residual magnetic flux density (Br) is 9.7 KG, and the (BH)max theoretical value of (Br/2)2 yields 23.5 MGOe. In contrast, the value of (BH)max in the patent document is 24.0 MGOe, plainly surpassing the theoretical value, so that a value that cannot in reality exist is cited in the patent document. Furthermore, the theoretical value is calculated based on ideal conditions with squareness of 100%, and in this case the squaring ratio of NdFeB anisotropic magnet powder and SmFeN anisotropic magnet powder is not more than about 40-70%. This sort of disclosure places the veracity of the information in that patent document in doubt. Moreover, in patent document 5, nothing is disclosed with respect to the heat resistance or irreversible loss ratio of the bonded magnet.
Incidentally, heat processing of ribbon fragments made by melt spinning method was performed on the NdFeB magnet powder used in each above-stated bonded magnet to make the powder anisotropic, but the anisotropy conferred was inadequate. Separately, a hydrogenation treatment process (HDDR process) which produces anisotropic magnet powder was developed. Composite rare-earth anisotropic bonded magnets using magnet powder made from this HDDR process (hereafter, “HDDR magnet powder”) are disclosed in patent documents 6-11 mentioned below.
In patent document 6 (Japanese patent application Laid-Open (Kokai) No. 9-92515), a bonded magnet is disclosed in which (1) HDDR magnet powder, including Co, with an average grain diameter of 150 μm, having an aggregate structure of re-crystallized grains comprised of Nd2Fe14B tetragonal phase, and (2) 0-50 wt % ferrite magnet powder comprised of SrO.6Fe2O3 with an average grain size of 0.5 to 10.7 μm, and (3) 3 wt % of epoxy resin are mixed at room temperature, vacuum dehydrated, molded within a magnetic field and heat-hardened.
Here, the above-mentioned Co is a necessary element for conferring anisotropy on the above-mentioned HDDR magnet powder. Further, by including Co, the temperature properties of HDDR magnet powder are improved, and the heat resistance of the bonded magnet increases. This was also introduced in non-patent document 1.
The bonded magnet disclosed in the embodiments of patent document 6 shows excellent magnetic properties and heat resistance, for example maximum energy product (BH)max 132-150.14 kJ/m3, and irreversible ageing loss (100° C.×1000 hours) −3.5 to −5.6%. However, these magnetic properties are not much different from those of material molded with the above-mentioned Co-containing HDDR magnet powder simple. In other words, the merits of a composite magnet powder are not expressed in the magnetic properties.
Patent document 6 explains the advantages of making a bonded magnet by mixing two types of magnet powder with different grain diameters as follows. When molding a bonded magnet, the result of having ferrite magnet powder preferentially fill the grain gaps of NdFeB magnet powder which is HDDR magnet powder is that the air gap percentage will decrease. In this way, (a) intrusion of O2 and H2O into the bonded magnet is controlled, improving heat resistance; (b) parts that were air gaps are permutated by ferrite magnet powder, improving magnetic properties; and (c) as a result of the ferrite magnet powder mitigating the stress concentration on the NdFeB magnet powder generated when molding the bonded magnet, fracturing of the NdFeB magnet powder is controlled. Thereby, exposure of exceptionally active fractured metal surfaces in the bonded magnet is controlled, and the heat resistance of the bonded magnet is further improved. Moreover, by mitigating the stress concentration with ferrite magnet powder, the importing of deformations into the magnet powder is controlled, further improving magnetic properties.
This patent document mentions that a decrease in irreversible loss rate (lowering heat resistance) is caused by fractures in the magnet powder, but also states that a surfactant does not have the effect of improving heat resistance, and there is no example embodiment using a surfactant.
In patent document 7 (Japanese patent application Laid-Open (Kokai) No. 9-115711) a bonded magnet is disclosed which uses, in place of the ferrite magnet powder of above-mentioned patent document 6, isotropic nano-composite magnet powder with an average grain diameter of 3.8 μm, comprised of (1) soft magnetic phase including body-centered cubic iron with average crystalline grain diameter 50 nm or less and iron boride, and (2) hard magnetic phase having Nd2Fe14B-form crystal. This bonded magnet has a maximum energy product (BH)max of 136.8 to 150.4 kJ/m3. The magnetic properties are more or less improved over patent document 6, but still insufficient. Although the bonded magnet has excellent heat resistance with irreversible loss rate −4.9 to −6.0%, this depends on the inclusion of Co.
Patent document 7 also discloses, as a comparison example, a bonded magnet which is made of Co-containing NdFeB magnet powder and SmFeN magnet powder with a smaller grain diameter than that of the NdFeB powder. This bonded magnet, although it has a maximum energy product (BH)max of 146.4 to 152.8 kJ/m3 and initial magnetic properties are excellent, irreversible loss rate is −13.7 to −13.1%. Heat resistance is worse than in bonded magnets made from Co-containing NdFeB magnet powder simple (irreversible aging loss rate: −10.4 to −11.3%).
Patent document 7 attributes that problem to oxidation of the SmFeN magnet powder. As a result, the idea of making a composite with SmFeN magnet powder in order to improve the heat resistance of bonded magnets made from Co-containing HDDR magnet powder was abandoned. Below-mentioned patent documents 8 through 11 make this clear.
In patent document 8 (Japanese patent application Laid-Open (Kokai) No. 9-312230), patent document 9 (Japanese patent application Laid-Open (Kokai) No. 9-320876), patent document 10 (Japanese patent application Laid-Open (Kokai) No. 9-330842), and patent document 11 (Japanese patent application Laid-Open (Kokai) No. 10-32134), a bonded magnet is disclosed which makes a composite of Co-containing HDDR magnet powder and another magnet powder (ferrite magnet powder, nano-composite, melt spun NdFeB magnet powder, etc.) with a grain diameter smaller than that of the HDDR powder. These bonded magnets are made by mixing each magnet powder at a normal temperature, and then within a temperature range above the softening point of the heat-hardened resin and below the point where hardening begins, molding within a magnetic field while at temperature. By molding within a magnetic field at temperature, magnet powder fluidity improves, and as a result of the filling factor of the whole and mitigating stress concentration between grains of magnet powder, the obtained bonded magnet exhibits excellent magnetic properties and heat resistance, with a maximum energy product (BH)max of 142.5 to 164.7 kJ/m3 and irreversible loss rate of −2.6 to −4.7%.
However, when looking at the amount of improvement in maximum energy product (BH)max due to using composite magnet powder for each fine powder individually, compared to Co-containing HDDR magnet powder simple, composite ferrite magnet powder shows improvement of 5.1-5.3%, composite melt spun NdFeB magnet powder improvement of 9.3 12.7%, and a composite of melt spun NdFeB magnet powder and Sr ferrite magnet powder shows improvement of 5.0 5.6%. In all cases the improvement in magnetic properties is small. Regardless of ample improvement in irreversible loss rate, the lack of improvement in maximum energy product (BH)max is thought due to the fact that the magnetic properties of the above-mentioned magnetic powder used for making a composite are quite inferior to the primary Co-containing HDDR magnet powder.
Co is a necessary element in the Co-containing HDDR magnet powder used in the above-stated patent documents 6-11, but it is widely known that because Co is a scarce resource, it is costly and not in steady supply. Accordingly, the above-stated Co-containing HDDR magnet powder is not desirable when aiming at enlarged demand for bonded magnets. Development of a bonded magnet using Co-less anisotropic magnet powder, while providing magnetic properties and heat resistance the same or greater as a magnet using Co-containing anisotropic magnet powder, is much desired.
The present invention develops a new hydrogenation process, the d-HDDR process, in place of the above-mentioned HDDR process, and despite not containing Co, succeeds at making anisotropic RFeB magnet powder. The contents of this d-HDDR process, by way of example, are specifically disclosed in patent document 12 (Japanese patent application Laid-Open (Kokai) No. 2001-76917). The contents of this process will also be stated later in the present specification.
The bonded magnet comprised of anisotropic magnet powder simple (hereafter, “d-HDDR anisotropic magnet powder”) made through this process has a maximum energy product (BH)max of 137.7-179.1 kJ/m3. It presently displays the highest magnetic properties of any bonded magnet made from Co-less magnet powder.
When d-HDDR anisotropic magnet powder does not contain Co, the oxidation resistance effect provided by Co can not be expected. Furthermore, constituent grains of the d-HDDR anisotropic powder are easily fractured during bonded magnet molding, because this powder has a higher sensitivity to fracturing than melt spun magnet powder due to having cracks generated at the time of hydrogen pulverization. When fractures occur in the constituent grains, the fracture surface is markedly oxidized, and the irreversible loss rate of the bonded magnet greatly deteriorates. Specifically, even though molded at temperature within a magnet field, bonded magnets comprised of Co-less d-HDDR anisotropic magnet powder alone, as an example, have irreversible loss rates (100° C.×1000 hr) no better than −23.0 to −18.0% when coercive force is 880-1040 kA/m. In particular, for the 120° C.×1000 hr called for in automotive environments, irreversible loss rate is notably worse at −28.0 to −35.0%. The present invention was made with this information in mind.
More specifically, the present invention furnishes a composite rare-earth anisotropic bonded magnet using Co-less d-HDDR anisotropic magnet powder and a method for its production; the magnet has high initial magnetic properties and provides ample heat resistance the same or greater than bonded magnets using Co-containing HDDR magnet powder. Further, the present invention furnishes a composite rare-earth anisotropic bonded magnet that provides ample heat resistance at temperatures of 120° C. and a method for its production. Also, the present invention furnishes, as raw material for such a bonded magnet, an ideal compound for a composite rare-earth anisotropic bonded magnet and a method for producing the compound.
Patent Document 1:    U.S. Pat. No. 4,851,058
Patent Document 2:    U.S. Pat. No. 5,411,608
Patent Document 3:    Japanese patent application Laid-Open (Kokai) No. 5-152116
Patent Document 4:    Japanese patent application Laid-Open (Kokai) No. 6-61023
Patent Document 5:    Japanese patent application Laid-Open (Kokai) No. 6-132107
Patent Document 6:    Japanese patent application Laid-Open (Kokai) No. 9-92515
Patent Document 7:    Japanese patent application Laid-Open (Kokai) No. 9-115711
Patent Document 8:    Japanese patent application Laid-Open (Kokai) No. 9-312230
Patent Document 9:    Japanese patent application Laid-Open (Kokai) No. 9-320876
Patent Document 10:    Japanese patent application Laid-Open (Kokai) No. 9-330842
Patent Document 11:    Japanese patent application Laid-Open (Kokai) No. 10-32134
Patent Document 12:    Japanese patent application Laid-Open (Kokai) No. 2001-76917
Non-Patent Document 1:    Journal of Alloys and Compounds 231 (1995) 51-59 (particularly, pgs. 54-55)